1. Field of the Invention
The present invention relates to a heat-shrinkable polyolefin composite sheet and particularly relates to a sheet for use in labels.
2. Prior Art of the Invention
Foamed sheets which have the property of being heat-shrinkable are widely used as pre-labels for winding on vessels such as glass bottles prior to filling the vessels. ParticularlY foamed polystyrene sheets are excellent in shock absorbing, thermal insulation and beautiful appearance, and additionally outstanding in wrapping for heat-winding on the vessels. Hence foamed polystyrene sheets are used in a large amount.
However, foamed polystyrene sheets exhibit low resistance to heat. They are unsuitable for labels of, for example, foods and drinks such as coffee and oolong tea which require increased temperature sterilization treatment including a retort sterilization step.
On the other hand, polyolefin base non-foamed sheets and films have been widely known to be suitable for retort sterilization and have been investigated for label materials having shrinkability. However, the sheets have problems in that the sheets do not constantly give cylindrical sleeves in the molding of prelabels, or the sleeves obtained do not uniformly shrink when heated and lead to deformation of the pattern printed thereon.
Further, heat-shrinking temperatures of polyolefin-base sheets and films are close to their melting temperatures. Shrunk sheets and films have poor slippage on the vessels and it is very difficult to obtain uniform patterns as in the case of heating the labels composed of heat-shrinkable polystyrene sheets.
There is also a problem that, even though labels are firmly adhered to the vessels, the labels consisting of non-foamed polyolefin-base sheets or films become loose when the temperature of the vessels returns to the room temperature and thus firm adhesion of the labels is impaired.
On the other hand, heat-shrinkable foamed polyolefin-base sheets have also been investigated. Foamed polyolefin sheets have good shrinkability. However, the sheets are opaque, and hence printing must be made on the outside surface. Outside printed sheets cause problems such as transfer of the printing ink and loss of the printed pattern due to rubbing. The sheets also have a disadvantage that the heat-sealing strength is insufficient and the sealing ability is unstable in the formation of sleeves. In the steps for heating and shrinking the sleeves so that they adhere to the vessels, countermeasures such as the use of hot-melt adhesives has also been examined in order to stabilize sealing. However, restrictions are imposed upon the use of the resulting vessels.
As mentioned above, vessels which require increased temperature treatment such as retort sterilization tend to exhibit ready separation of the sealed portions. The cells on the outside surface of the foamed sheets are also impaired in the increased temperature treatment step and broken cells contain moisture. On account of these problems, suitable prelabel sheets which can satisfy conditions in the increased temperature treatment such as retort sterilization have not yet been developed.